Frequency converters each comprised of a three-phase rectifier and an inverter have been widely used for adjustable speed drive control for an air-conditioner system with the development of semiconductor elements and microcomputers, resulting in the ability to readily obtain precise control. On the other hand, however, when precise control is effected by such equipment, many higher harmonic voltage or current components, occur in circumstance where such higher harmonic voltages or currents exert, adverse influence on other equipment. Thus, measures for reducing such higher harmonic voltages or currents have been required. The management standard for such a higher harmonic problem of this kind is uncertain because of the difficulty in determining its cause. In addition, the scope of responsibility of electric power companies and customers is not clearly defined. Thus, various problems exists.
Typical frequency converters are composed of a three-phase rectifier consisting of diodes, a smoothing capacitor, and a three-phase inverter. The input current of such frequency converters has a waveform different from a normal sine wave, i.e., a waveform including higher harmonics, particularly the fifth higher harmonic component.
For the purpose of reducing higher harmonic components included in load current in the case where such frequency converters are used, e.g., in power supply equipment for an air-conditioner system installed in a building, there has been proposed a system in which two transformers with different connections, e.g., a transformer with delta-delta connection and a transformer with delta-star connection are used to provide two sets of a.c. voltages having a phase difference of 30 degrees to connect frequency converters to their output sides, respectively. In the case of this system, which can be called a phase difference transformer system, input currents to individual frequency converters have distorted waveforms including the above-described higher harmonics, respectively. However, current obtained by synthesizing both input currents, i.e., load current as viewed from the power supply has a waveform closer to a sine wave as a result of compensatory summation of respective distortions, i.e., a waveform including lower harmonic components.
The above-described phase difference transformer system which is known as a measure for reducing higher harmonics, has the advantage that substantially the same configuration is used for both frequency converters, but the drawback that two sets of transformers for the rectifiers are required, resulting in an increased cost.